Distribution of digitized composite AM FDM signals

ABSTRACT

Signals, such as television signals, are distributed from a head-end station via a fibre network. The signals are amplitude modulated on carriers at different frequencies, and the different carriers combined to form a composite analogue signal. In the head-end station the composite analogue signal is clipped to reduce the peak-to-mean ratio and the clipped composite signal is digitized. The resulting digital data is transmitted onto the fibre network and is received at an optical receiver. The optical receiver reconstitutes the composite analogue signal for reception by a termination. In one example the composite analogue signal has a bandwidth of one octave or less, and is digitized by sampling at a sampling rate below the Nyquist rate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the distribution of signals on anetwork and in particular to the distribution of AM (amplitudemodulated) television signals.

2. Related Art

Conventionally, television has been broadcast as AM signals withdifferent channels modulated onto sub-carriers at different frequencies.Existing cable television services using copper coaxial cable haveadopted corresponding analogue AM techniques to provide channels in aregion of the same UHF spectrum used for broadcast television.

It is now proposed to use cable networks such as fibre optic networksfor the distribution of television signals. The wide bandwidth offeredby such networks offers the possibility of upgrading to carry futurewideband services such as HDTV and the same network may also be used forservices other than television, such as telephony. However whilst theuse of AM techniques is necessary if the television signals are to bereceived by conventionally equipped television sets there are a numberof problems associated with the use economically of AM techniques onoptical networks. In particular the carrying of AM optical signalsrequires less splitting in order to allow a high power budget, and theuse of highly specified linear opto-electronic devices.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, a method of distributing signalsfrom a head-end station via a network comprises combining a plurality ofAM channels modulated on sub-carriers at different frequencies to form acomposite analogue signal, and characterised by treating the compositeanalogue signal to reduce the peak-to-mean ratio, digitizing thecomposite analogue signal, transmitting resulting digital data onto thenetwork, receiving the digital data at a receiver and reconstituting thecomposite analogue signal for reception at a termination.

Preferably the network is an optical network and the receiver is anoptical receiver. Preferably the composite analogue signal is treated byclipping the signal.

A preferred aspect of the present invention provides a method ofdistributing, e.g. , television signals which is particularlywell-adapted to use with optical networks whilst at the same timemaintaining compatibility with conventional analogue television sets. Asnoted above, television signals used for terrestrial broadcasting aremodulated onto sub-carriers at different frequencies. When a compositesignal is formed by adding together the different channels the resultingwaveform has a much higher peak-to-mean ratio than the individualcarriers. It is therefore possible to clip or compress the maxima andminima before quantizing without significantly degrading theperformance. The A/D converter used for digitizing the signal can thenoperate over a more restricted input amplitude range and so requiresfewer quantization levels to achieve an adequate output videosignal-to-noise ratio.

Preferably the method includes receiving the digitized signal at a noderemote from the termination and transmitting the reconstituted signalonwards for reception at the termination.

The detection of the digital data is advantageously carried out at anode such as a distribution box or pedestal near the customer'spremises. The television signals may then be delivered over conventionalcoaxial cable in a standard format, avoiding the need for a specialadapter or other equipment at the customer-end.

Preferably the composite analogue signal has a bandwidth of one octaveor less and is digitized by sampling at a sampling rate below theNyquist rate.

Preferably, in addition to or as an alternative to clipping thecomposite analogue signal the method further comprises compressing thecomposite analogue signal at the head-end station and applyingcomplementary expansion at the receiver.

Companding may be carried out either in the analogue or in the digitaldomain and preferably the compression function is the cumulativedistribution function of the Gaussian probability distribution function.

According to a further aspect of the present invention there is provideda system for distributing signals comprising a head-end station, one ormore receivers, and a network connecting the head-end station to the oreach receiver, and characterised by the head-end station including aninput stage arranged to receive a composite analogue signal comprising aplurality of AM channels modulated on sub-carriers at differentfrequencies, converter means arranged to treat the composite analoguesignal to reduce the peak-to-mean ratio and to digitize the compositeanalogue signal, and transmitter means arranged to transmit theresulting digital data onto the network, the receiver includingreconstituting means arranged to reconstitute the composite analoguesignal and output means arranged to output the composite analogue signalfor reception by a termination.

A method and system in accordance with the present invention will now bedescribed in further detail with reference to the accompanying drawings,in which:

FIG. 1 is a block diagram showing a distribution system;

FIG. 2 is a block diagram showing a signal path in greater detail;

FIG. 3 is a diagram showing an alternative arrangement for themultiplexer of FIG. 2; and

FIGS. 4a-4d are diagrams showing sub-carriers and a composite signal.

A system for distributing television signals via an optical fibrenetwork comprises a head-end station 1 and a number of optical receivers2 connected to the head-end station 1 via a fibre network 3. In thepresent example the fibre network 3 is a passive optical network using astar topology but the present invention is equally applicable to otherfibre optic networks using different topologies.

The head-end station 1 receives at its input a composite analogue signalwhich in the present example covers the spectrum from 470-860 MHz. Thisinput signal comprises sub-carriers at different frequenciesamplitude-modulated with different television signals. These signals areinput to an A/D converter 4 which, as described in further detail below,clips and digitises the signal. An optical transmitter 5 modulates anoptical signal with the digital data output by the A/D converter andoutputs the signal onto the fibre network 3. The signal passes throughdifferent stages of the fibre network 3, including one or more passiveoptical splitters 7 and is received by the optical receivers 2. Forclarity only two optical receivers 2 are shown in FIG. 1, but inpractice many more may be connected to each network. At the receivers 2which may, for example, be positioned in street cabinets, the compositeoptical signal is reconstituted from the optical data and output as ananalogue signal via coaxial cables 8 to conventional television sets 13.

The signal path from the head-end station 1 to the television set isshown in greater detail in FIG. 2. Data is output from the A/D converter4 in parallel as 8-bit words. An 8:1 multiplexer 18 converts the outputfrom the A/D converter 4 to a serial bit stream. This bit stream is usedto modulate the output from the optical transmitter 5 using conventionaltechniques. The operation of the multiplexer 18 is locked to a 7 GHzclock generated locally in the head-end station. This clock signal isdivided by eight to provide a lower rate clock for the A/D converter 4.In transmission over the fibre network 3 the signal output by thetransmitter 5 may be split up to 49 ways, within the constraints of thepower budget in the present example. At the receiver 2 a 1:8demultiplexer 12 assembles 8-bit parallel words from the serialdatastream and outputs those 8-bit words to a D/A converter 9. Theoutput from the D/A converter 9 passes through a band pass filter 10 andvia an output amplifier stage 11 to up to 20 different television sets13 connected to the receiver 2 by conventional coaxial cables.

The system may be used to distribute signals to so-called "cable ready"sets. Such sets typically have a VHF baseband input up to 450 MHz. Forsuch systems use can be made of aliasing channels present in the lowerfrequency range. These are channels at frequencies below those of theoriginal components of the composite analogue signal which are generatedas a result of sampling below the Nyquist rate. In this case thereconstituting filter 10 is a low-pass filter rather than a band-passfilter. The low frequency channels output from the reconstituting filterare inverted unless the input to the A/D converter is first inverted.

FIGS. 4a-4d, show schematically the formation of the composite analoguesignal from three carriers c₁, c₂, c₃. The carriers have amplitudes p₁,p₂, p₃ respectively. The mean (rms) level of each carrier is then p_(n)/(2√2). The different component carriers add together to form thecomposite signal shown in FIG. 4d. It can be shown that the amplitude ofthe composite signal, when it comprises n channels, each of amplitude pis np and its mean level is n(p/2√2). The peak-to-mean ratio of thecomposite signal is therefore far greater than that of the individualsub-carriers. At the input to the A/D converter relatively severeclipping may be applied to limit the dynamic range of the signal priorto quantization making it possible to minimize the number ofquantization levels without significant loss in the quality of thesignal. Clipping may be applied using an appropriate clipping stagebefore the A/D converter. Alternatively if that converter has suitableclipping characteristics clipping may be achieved by overloading it by asuitable margin. Any suitable conventional clipping function can beemployed as is in common in signal quantization systems. See, forexample, Bylanski et al, "Digital Transmission Systems", chapter 2 on"Digital-system impairments", pp. 26-35, Institution of ElectricalEngineers, 1976.

The composite analogue signal is limited in bandwidth to less than oneoctave. Not only does this serve to reduce second order intermodulationdistortion but it makes possible sampling at a rate below the Nyquistminimum, that is twice the frequency of the maximum frequency componentof the signal. Sampling below the Nyquist rate causes aliasingdistortion. When however the signal is confined to less than one octaveand sampled at a rate only slightly greater than the maximum frequencycomponent the aliasing distortion falls in the unused frequency bandbelow the signal octave band. The aliasing distortion therefore does notcompromise the performance of the system and the bit rate of the systemis only one half that needed for full Nyquist sampling. As noted above,when generating signals for cable-ready VHF TV sets the production oflow frequency aliasing channels may be used to shift the receivedchannels into the desired frequency range.

The A/D converter 4 defines a number of amplitude bands and determineswhich amplitude band matches the sample value. Each amplitude band isrepresented by a binary word and a byte is output accordingly. Theresultant bit rate when these bytes are converted to a serial stream bythe 8:1 multiplexer 8, is the product of the sampling rate and thenumber of digits in a byte. Clipping minimizes the number ofquantisation levels required and so enables a reduction in the byte sizeand bit rate.

Optionally compression may be applied to the signal before or after itis sampled by the A/D converter 4. Known A/D converters for videosignals use linear quantizing with one output byte representing onequantization level. The present system however by applying compressionto the composite analogue signal as it is digitized reduces the numberof possible output words required for a given output video quality by afactor of two or more, giving a further saving in the bit rate. Thepreferred compression function is the cumulative distribution functionof the Gaussian probability distribution function (PDF). If the inputwaveform to the compressor has a Gaussian PDF, the output PDF from thecompressor is uniform with output in the range 0 to 1. This uniform PDFcan then be quantised linearly and each output codeword is thenequi-probable. The compressor and matching expander at the receiver maybe realized in hardware using a network of diodes and resistors, or insoftware using a look-up table following a suitable linear A/Dconverter.

The digital circuits in the head-end station 1 and the receiver 2 aresynchronized using word alignment techniques. For example, the leastsignificant bit in each word may be identified by a periodic functionsuch as a Barker code by regular bit stealing. This sequence is thendetected using a suitable sampler, logic function and clock slipcircuit. Preferably the clock operates at the word rate rather than theserialised bit rate.

In an alternative arrangement shown in FIG. 3 two 8:1 multiplexers areused in parallel at a lower clock rate of 1.2 GHz. In this case a pairof A/D converters are used, operating at a clock rate one eighth of thatof the multiplexers.

Although the embodiment discussed above relates to the distribution oftelevision signals, the invention, in its broadest aspects, isapplicable to other signals such as, e.g., audio signals.

We claim:
 1. A method of distributing signals from a head-end stationvia a network comprising:combining a plurality of AM channels modulatedon sub-carriers at different frequencies to form a composite analoguesignal, treating the composite analogue signal to reduce thepeak-to-mean ratio, digitizing the composite analogue signal,transmitting resulting digital data onto the network, and receiving thedigital data at a receiver and reconstituting the composite analoguesignal for reception at a termination.
 2. A method according to claim 1,in which the network is an optical network, the receiver is an opticalreceiver, and the composite analogue signal is treated to reduce itspeak-to-mean ratio by clipping the signal.
 3. A method according toclaim 1, including:receiving the digitized signal at a node remote fromthe termination, and transmitting the reconstituted signal onwards forreception at the termination.
 4. A method according to claim 1,including compressing the composite analogue signal at the head-endstation and applying complementary expansion to the signal at thereceiver.
 5. A method according to claim 1, in which the signalsdistributed from the head-end station are television signals.
 6. Amethod of distributing signals from a head-end station via a networkcomprising:combining a plurality of AM channels modulated onsub-carriers at different frequencies to form a composite analoguesignal, treating the composite analogue signal to reduce thepeak-to-mean ratio, digitizing the composite analogue signal,transmitting resulting digital data onto the network, and receiving thedigital data at a receiver and reconstituting the composite analoguesignal for reception at a termination, the composite analogue signalhaving a bandwidth of one octave or less and being digitized by samplingat a sampling rate below the Nyquist rate.
 7. A method as in claim 6, inwhich the network is an optical network, the receiver is an opticalreceiver, and the composite analogue signal is treated to reduce itspeak-to-mean ratio by clipping the signal.
 8. A method as in claim 6including:receiving the digitized signal at a node remote from thetermination, and transmitting the reconstituted signal onwards forreception at the termination.
 9. A method as in claim 6, includingcompressing the composite analogue signal at the head-end station andapplying complementary expansion to the signal at the receiver.
 10. Asystem for distributing signals comprising:a head-end station, one ormore receivers, a network connecting the head-end station to the or eachreceiver, the head-end station including an input stage arranged toreceive a composite analogue signal comprising a plurality of AMchannels modulated on sub-carriers at different frequencies, convertermeans arranged to treat the composite analogue signal to reduce thepeak-to-mean ratio and to digitize the composite analogue signal, andtransmitter means arranged to transmit the resulting digital data ontothe network, and the receiver including reconstituting means arranged toreconstitute the composite analogue signal and output means arranged tooutput the composite analogue signal for reception by a termination. 11.A system according to claim 10, in which the network is an opticalnetwork, the receiver is an optical receiver, and the converter meansare arranged to treat the composite analogue signal by clipping it. 12.A system according to claim 11, in which the or each optical receiver islocated at a respective node remote from the termination and is arrangedto transmit the reconstituted signal onwards for reception at thetermination.
 13. A system according to claims 10, further includingcompression means located in the head-end station and expansion meanslocated in the or each receiver and arranged to apply complementarycompression and expansion to the composite analogue signals.
 14. Asystem for distributing signals comprising:a head-end station, one ormore receivers, a network connecting the head-end station to the or eachreceiver, the head-end station including an input stage arranged toreceive a composite analogue signal comprising a plurality of AMchannels modulated on sub-carriers at different frequencies, convertermeans arranged to treat the composite analogue signal to reduce thepeak-to-mean ratio and to digitize the composite analogue signal, andtransmitter means arranged to transmit the resulting digital data ontothe network, and the receiver including reconstituting means arranged toreconstitute the composite analogue signal and output means arranged tooutput the composite analogue signal for reception by a termination, thecomposite analogue signal having a bandwidth of one octave or less andthe converter means being arranged to sample the composite analoguesignal at a sampling rate below the Nyquist rate.
 15. A system accordingto claim 14, in which the network is an optical network, the receiver isan optical receiver, and the converter means are arranged to treat thecomposite analogue signal by clipping it.
 16. A system as in claim 15,in which the or each optical receiver is located at a respective noderemote from the termination and is arranged to transmit thereconstituted signal onwards for reception at the termination.
 17. Asystem as in claim 14, further including compression means located inthe head-end station and expansion means located in the or each receiverand arranged to apply complementary compression and expansion to thecomposite analogue signals.
 18. A system as in claim 14, in which thesignals distributed from the head-end station are television signals.